Technetium reduction in sediments of a shallow aquifer exhibiting dissimilatory iron reduction potential

Abstract

Pertechnetate ion [Tc(VII)O(4) (-)] reduction rate was determined in core samples from a shallow sandy aquifer located on the US Atlantic Coastal Plain. The aquifer is generally low in dissolved O(2) (<1 mg L(-1)) and composed of weakly indurated late Pleistocene sediments differing markedly in physicochemical properties. Thermodynamic calculations, X-ray absorption spectroscopy and statistical analyses were used to establish the dominant reduction mechanisms, constraints on Tc solubility, and the oxidation state, and speciation of sediment reduction products. The extent of Tc(VII) reduction differed markedly between sediments (ranging from 0% to 100% after 10 days of equilibration), with low solubility Tc(IV) hydrous oxide the major solid phase reduction product. The dominant electron donor in the sediments proved to be (0.5 M HCl extractable) Fe(II). Sediment Fe(II)/Tc(VII) concentrations >4.3 were generally sufficient for complete reduction of Tc(VII) added [1-2.5 micromol (dry wt. sediment) g(-1)]. At these Fe(II) concentrations, the Tc (VII) reduction rate exceeded that observed previously for Fe(II)-mediated reduction on isolated solids of geologic or biogenic origin, suggesting that sediment Fe(II) was either more reactive and/or that electron shuttles played a role in sediment Tc(VII) reduction processes. In buried peats, Fe(II) in excess did not result in complete removal of Tc from solution, perhaps because organic complexation of Tc(IV) limited formation of the Tc(IV) hydrous oxide. In some sands exhibiting Fe(II)/Tc(VII) concentrations <1.1, there was presumptive evidence for direct enzymatic reduction of Tc(VII). Addition of organic electron donors (acetate, lactate) resulted in microbial reduction of (up to 35%) Fe(III) and corresponding increases in extractable Fe(II) in sands that exhibited lowest initial Tc(VII) reduction and highest hydraulic conductivities, suggesting that accelerated microbial reduction of Fe(III) could offer a viable means of attenuating mobile Tc(VII) in this type of sediment system.

Retention of99mTc at Ultra-trace Levels in Flowing Column Experiments – Insights into Bioreduction and Biomineralization for Remediation at Nuclear Facilities

“…The medical radiotracer 99m Tc (half-life 6 h), allows high resolution imaging of sediments containing ultra-trace concentrations of 99m Tc (typically < 5 £ 10 ¡10 mol l ¡1 ) making it possible to conduct studies at technetium concentrations typical of far field contamination at nuclear sites down gradient of the contaminant plume source Corkhill et al 2013;Lear et al 2010;Vandehey et al 2012), and below the theoretical solubility limit of hydrous TcO 2 (»10 ¡8 mol l ¡1 ; Hess et al 2004). At these environmentally relevant concentrations of contaminant, bioavailable Fe will be in stoichiometric excess in typical bioreduced sediments and the rate of Tc(VII) reduction, and the mobility of 99 Tc will be controlled by the Fe(II) speciation Peretyazhko et al 2012;Wildung et al 2004).…”

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides. 3. Influence of Chemical Speciation and Bioavailability on Contaminants Immobilization/Mobilization Bio-processes

“…Some DMRB can reduce solid phase Fe(III) oxides and oxyhydroxides including poorly crystalline phases such as ferrihydrite and crystalline phases such as goethite, hematite, and magnetite (Fredrikson et al 2000;Behrends & van Cappellen 2005). The roles of biogenic Fe(II) in the reductive immobilization and potential use for the in situ immobilization of redox-sensitive contaminants such as technetium (Fredrikson et al 2004;Wildung et al 2004), selenium (Zingaro et al 1997), plutonium ) and uranium (Fredrikson et al 2000;Behrends & van Cappellen 2005) has been lately studied.…”

“…It is highly mobile in its oxic form (as Tc(VII)O 4 ) ) but is scavenged to sediments in its reduced forms (predominantly Tc(IV)). Wildung et al (2004) studied the extend of pertechnetate ion [Tc(VII)O 4 ) ] reduction in sediments. The dominant electron donor in the sediments proved to be Fe(II).…”

Redox interactions of technetium with iron-bearing minerals

“…Further, bioreduction (where indigenous sediment microorganisms are stimulated to produce reducing conditions by the addition of an electron donor) has been shown to be an effective means of removing Tc(VII) from solution in microcosm (Burke et al, 2005;Begg et al, 2007;Law et al, 2010a), column (Michalsen et al, 2006;Lear et al, 2010) and field studies (Istok et al, 2004) over a wide range of conditions. A typical feature in these bioreduction experiments is that the removal of Tc(VII) is associated with the development of microbially-mediated Fe(III) reduction in the sediments (Burke et al, 2005;Fredrickson et al, 2004;Li and Krumholz, 2008;Michalsen et al, 2006;Wildung et al, 2004). Furthermore, removal is thought to be dominated by abiotic electron transfer from sediment associated biogenic Fe(II) to Tc(VII) in all but Fe-poor environments (Lloyd et al, 2000;Begg et al, 2007;Fredrickson et al, 2004;Plymale et al, 2011;Law et al, 2010a;Zachara et al, 2007).…”